JP4167778B2 - Method for evaluating aberration of optical element - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method and apparatus for evaluating aberration of an optical element such as an optical head for an optical recording / reproducing apparatus.
[0002]
BACKGROUND OF THE INVENTION
In general, conventional methods for evaluating the aberrations of optical elements require two steps. The first step is a step of reproducing the original wavefront from the sharing pattern of the two divided images. Next, the second step is a step of obtaining a plurality of aberrations from the reproduced wavefront.
[0003]
Specifically, FIG. 19 shows an apparatus 300 for determining the aberration of the optical element 302. When determining the aberration of the optical element, the light from the optical element 302 is sent to a first beam splitter 304 where it is split into a first light 306 and a second light 308. The first light 306 that has passed through the first beam splitter 304 is reflected by the first mirror 310 and then sent to the image receiving unit 314 via the second beam splitter 312. The second light 308 reflected by the first beam splitter 304 is reflected by the second mirror 316 and the second beam splitter 312 and sent to the image receiving unit 314. The second mirror 316 is arranged so that the first light 306 and the second light 308 are shifted from each other on the image receiving unit 314 to form a sharing interference image or a sharing pattern thereon. Next, the sharing interference image or the sharing pattern is analyzed by the image processor 316 to determine the aberration of the optical element.
[0004]
According to the above configuration, the original wavefront is obtained from the shearing interference image or the shearing pattern, where many steps are required to evaluate the aberration, which takes a lot of time. In addition, it is necessary to analyze a second order matrix, which requires a large number of calculation processes. Similarly, the calculation for obtaining the original wavefront from the shearing interference image requires a large number of man-hours and therefore takes time. In addition, the light is divided into two, and the two divided lights must be accurately superimposed on the image receiving unit 314. However, in order to do so, each light path must be stably maintained, which increases the size of the apparatus.
[0005]
FIG. 20 shows another conventional apparatus 318 for determining the aberration of the optical element to be adjusted. According to this apparatus, the light 322 is sent to the transparent plate 326 via the objective lens 324 of the optical element 320. Next, the light 322 is imaged on the image receiving unit 330 as a light spot by the converging lens 328, and the image receiving unit 330 forms a series of signals corresponding to the received image. This signal is sent to the signal processor 332, where the light intensity distribution of the received image is determined. The light intensity distribution is used to determine the aberration of the optical element 320, and the determined aberration is used to adjust the optical element 320.
[0006]
However, in this embodiment, the imaged light spot has to be greatly enlarged. Therefore, the field of view of the image receiving unit 328 is very narrow. This means that if the image spot is slightly deviated, the light spot is deviated from the field of view of the image receiving unit and aberration cannot be detected. Further, the spot light does not include phase information, and therefore it is difficult to accurately obtain the aberration.
[0007]
SUMMARY OF THE INVENTION
Accordingly, an object of the present invention is to provide an excellent method and apparatus capable of easily obtaining the aberration of an optical element.
[0008]
In accordance with the purpose, in the method of evaluating the aberration of the optical element, the light is transmitted through the optical element and then diffracted into diffracted light on the order of, for example, 0th order, ± 1st order, ± 2nd order, etc. . In particular, the first and second lights (for example, diffracted light of the order of 0th order and + 1st order, 0th order and −1st order, and −1st order and + 1st order) are superimposed, and these first and second lights are superimposed. A sharing interference image is formed. Next, the light intensity is detected at the first and second points in the sharing interference image. Here, the light intensity at the first point and the second point changes. Next, the phase difference of the light intensity between the first and second points is obtained. Using these phase differences, the aberration of the optical element is obtained.
[0009]
In another embodiment of the present invention, a plurality of points are determined in the sharing area. Specifically, the first to seventh points are determined. The first point is the center of the first line connecting the axes of the first diffracted light and the second diffracted light. The second point is located on a second line passing through the first line at the first point. The third point is located on the second line, and the second point and the third point are located symmetrically with respect to the first line. The fourth point and the fifth point are located symmetrically on both sides of the first line on the second line, and the fourth roll and the fifth point are separated from the first line by a predetermined distance, respectively. Yes. The sixth point and the seventh point are located on both sides of the first line, and the sixth and seventh points are separated from the first line by the predetermined distance.
[0010]
In another form of the invention, the method comprises a plurality of steps for determining the coma of the optical element. Here, the first phase difference Ph (1) of the light intensity between the first point and the second point is obtained. Similarly, the second phase difference Ph (2) of the light intensity between the second point and the third point, and the third phase difference Ph of the light intensity between the fourth point and the fifth point. (3) and the fourth phase difference Ph (4) of the light intensity at the sixth point and the seventh point is obtained. Using these phase differences, the magnitude of the coma aberration is obtained from the phase difference obtained from the following equation.
Phase difference = | Ph (1) −Ph (2) | / 2
Further, the coma aberration direction is obtained from the phase difference obtained from the following equation.
Phase difference = | Ph (4) −Ph (3) | / 2
In another form of the invention, the astigmatism of the optical element is determined. To determine astigmatism, the diffraction grating is oriented in three directions. For each direction, the light is sent through the optical element and guided through the diffraction grating to obtain first and second diffracted light. The second and third diffracted lights are superimposed on each other to form a shearing interference image. Next, the light intensity is obtained at the first and second points in the sharing interference image. The first point and the second point are located on a line passing through the midpoint of another line connecting the centers of the first diffracted light and the second diffracted light and symmetrically with the other line. In this state, the light intensity is changed. Also, the difference in light intensity between the first point and the second point is determined, and this is used to evaluate the astigmatism of the optical element.
[0011]
An apparatus for evaluating the aberration of an optical element has a reflective or transmissive diffraction grating. A plurality of grooves are formed in the diffraction grating, and light from the optical element is diffracted to obtain diffracted light. The diffracted light includes the first light and the second light, and the light partially overlaps to form a sharing interference image. A mechanism for moving the diffraction grating in a direction substantially perpendicular to the optical axis is provided. Next, the sharing interference image is received by the image receiving unit. The phase of the light intensity is obtained at each of a plurality of points in the sharing interference image and used for the evaluation of aberration.
[0012]
Another apparatus for evaluating aberrations of an optical element has a pair of first and second transmissive diffraction gratings. Each of the first diffraction grating and the second diffraction grating is formed with a plurality of parallel slits for diffracting light into diffracted light other than the 0th-order diffracted light. The first and second diffraction gratings are arranged in parallel to each other. The slit is oriented in a predetermined direction to form a shearing interference image in which two diffraction images are partially overlapped. A mechanism for moving the first diffraction grating in another direction that forms a predetermined angle with the predetermined direction is provided. In addition, an image receiving unit for receiving the sharing interference image and a processing device are provided for obtaining the phase of the light intensity at a plurality of points of the sharing interference image.
An apparatus for correcting the aberration of the optical element has a mechanism for correcting the aberration of the optical element.
[0013]
DETAILED DESCRIPTION OF THE INVENTION
DESCRIPTION OF EXEMPLARY EMBODIMENTS Hereinafter, preferred embodiments of the invention will be described with reference to the accompanying drawings.
[0014]
(1) First embodiment
FIG. 1 shows a first embodiment of an optical system 1 according to the invention for evaluating various aberrations in an optical element. Corresponding to this purpose, the optical system 1 has an optical device denoted by reference numeral 10. The optical apparatus 10 is an optical assembly or an optical head mounted on an optical recording / reproducing apparatus such as a digital video disk reproducing apparatus, and includes an optical element or an optical lens 12 supported by an appropriate support (not shown). Yes. The optical device 10 also has a light source 14 that emits light 16. The light source 14 preferably includes a laser source 14 that generates and emits a laser. For example, a modulator 18 composed of a plurality of optical elements is provided to modulate light into collimated light 20, and this modulated light is sent along the optical axis 22 of the lens 12. A beam splitter 24 is provided between the modulator 18 and the lens 12 on the optical axis 22.
[0015]
The collimated light passes through the beam splitter 24 and is sent to the lens 12. On the other hand, another light 26 incident on the beam splitter 24 in the opposite direction from the lens 12 is sent by the beam splitter 24 in another direction perpendicular to the collimated light 20.
[0016]
In order to receive the light 26 reflected by the beam splitter, the optical system 1 has an image receiving unit 28. The image receiving unit 28 includes a large number of light receiving elements constituting an imaging device (CCD), and each light receiving element converts the received image into a continuous image signal. The image receiving unit 28 is electrically connected to a signal processor (signal processing unit) 30. The signal processor 30 is also connected to an image display unit 32 having an image display unit 34 such as a CRT or LCD, and an image received by the image receiving unit 28 is reproduced on the display unit 34. The image display unit 32 is connected to an input device such as a keyboard and a mouse in order to draw a line on the displayed image and to specify a point in the displayed image.
[0017]
The optical system 1 includes a reflective diffraction grating 40. The diffraction grating 40 has a flat surface 42 on which many small grooves 44 are formed in parallel. The diffraction grating 40 may be part of an optical disc that is reproduced by the apparatus 10. The surface 42 is coated with a reflective thin film made of a suitable metal so that the surface 42 can reflect light.
[0018]
The diffraction grating 40 is supported by an appropriate support 46, and the surface 42 faces the lens 12 with a predetermined small gap. This gap is determined so that the collimated light 20 can be accurately focused by the lens 12 on the surface 42 of the diffraction grating 40.
[0019]
The diffraction grating support portion 46 is drivingly connected to a suitable drive mechanism 40 together with the diffraction grating 40, and can reciprocate in a reference direction indicated by an arrow 50 perpendicular to the optical axis 22 of the lens. The lens optical axis 22 can be rotated in the direction indicated by 52.
[0020]
If the optical system 1 only evaluates the aberrations of the lens 12, the system 1 is preferably designed so that the lens 12 is removable. On the other hand, when the device 10 is built in an optical device such as a DVD, the device 10 may be detachably provided in the optical system 1 for evaluating the aberration of the device.
[0021]
In the operation of the optical system 1 configured as described above, the diffraction grating 40 is disposed and fixed on the grating support 40. Next, the drive mechanism 48 is activated and the diffraction grating 40 is conveyed in the direction 50. Meanwhile, the light source 14 emits light 16. The emitted light is modulated into collimated light 20 by the modulator 18. Next, the collimated light 20 passes through the beam splitter 24, reaches the lens 12, and is imaged in a groove 38 that passes through the focal position of the lens 12. The imaged light is diffracted by the groove 38, reflected, and returned to the lens 12.
[0022]
The reflected light includes diffracted light of the order of 0th order, ± 1st order, ± 2nd order,. In the present embodiment, the diffraction angle of the diffraction grating 40 is designed so that diffracted light of the order of 0th order and ± 1st order is incident on the lens 12 and shares a part of the opening of the lens 12 or the pupil. ing. As will be apparent to those skilled in the art, the angle of diffraction can be determined by the wavelength of the incident light and the spacing of the grooves 44.
[0023]
The 0th order and ± 1st order diffracted lights interfere with each other to form interference fringes (a sharing interference image or a sharing pattern). This sharing interference image reflects various aberrations (described in detail below) included in the lens 12. Next, the 0th-order and 1st-order diffracted lights are collimated by the lens 12, reflected by the beam splitter 24, and sent to the light receiving unit 28. The light receiving unit 28 creates a signal corresponding to the received image. The signal is sent to the signal processor 30 and processed into an image signal displayed on the display unit 34.
[0024]
FIG. 2 shows a typical image displayed on the display unit 34. The displayed image has a sharing pattern 60. The sharing interference image 60 includes circular diffraction light images 62 and 64 on the order of 0th order and + 1st order, and these images are partially overlapped to form a sharing region 66. In the sharing pattern 60, alphabets (O) and (O ′) represent the centers of the circular images 62 and 64, respectively. The line indicated by reference numeral 68 indicates the sharing axis or shelling direction, and another line indicated by reference numeral 70 is perpendicular to the sharing axis and represents the midpoints of the centers (O) and (O ′) of the circular images 62 and 64. Indicates a crossing line or direction. Note that, due to the rotation of the diffraction grating 40 with respect to the reference direction 50, the sharing pattern 60 becomes sharing Together with the shaft 66, it rotates on the display 34 as shown in FIGS. 3B and 3C. In these figures, θ2 and θ3 are relative to the reference direction 50. sharing The rotation angle of the shaft 68 is shown.
[0025]
The light intensity at an arbitrary point in the sharing region 66 changes as the diffraction grating 40 moves. Further, the change in the light intensity at a certain point in the sharing region 66 has a specific phase, which is different from the phase at another point. Therefore, in order to evaluate the aberration of the lens 12, the phase difference of the light intensity between the two selected points is used. This will be described in detail later.
[0026]
Next, aberration evaluation of the apparatus 10 will be described. As described above, the aberration is evaluated by detecting the phase difference of the light intensity at a plurality of selected points in the sharing region 66. In order to obtain the phase difference of the light intensity, a phase shift method is adopted. In this phase shift method, the light intensity is detected at a plurality of predetermined points, and the diffraction grating moves in the reference direction during that time. This phase shift method is described in detail in (Optical Shop Testing, ed. D. Malacara (John Wiley and Sons, New York, 1978), p.414.
[0027]
In order to understand the aberration evaluation according to the present invention, each of the aberrations handled in the present invention will be briefly described. Specifically, FIGS. 4A to 7C show wavefronts including defocus, spherical aberration, coma and astigmatism caused by sharing interference of two diffracted images. In these drawings, λ indicates the phase of the light received by the image receiving unit 28. Θ represents an angle formed by the sharing axis and the reference direction.
[0028]
First, in FIGS. 4A to 4C, a defocus wavefront including an interference fringe generated by defocusing as shown by a solid line appears symmetrically with respect to the sharing axis. The wavefront due to defocusing can be expressed by the coordinates of the ξ axis and the η axis by the following equation (1).
Фdefocus = K (ξ ² + Η ² (1)
Фdefocus: Defocused wavefront function
K: Constant
[0029]
This equation shows that the interference fringes caused by the sharing of the diffracted image extend perpendicular to the sharing axis. That is, the light intensity change at one point on one side of the sharing axis has the same phase as the phase of another point provided symmetrically on the opposite side of the sharing axis.
[0030]
Next, as shown in FIGS. 5A to 5C, a wavefront represented by a solid line and including interference fringes caused by spherical aberration appears symmetrically with respect to the sharing axis. The wavefront caused by spherical aberration can be expressed by the coordinates of the ξ axis and the η axis by the following equation (2).
Фspherical aberration = Q （ξ ² + Η ² ) ² (2)
Фspherical aberration: function of wavefront due to spherical aberration
Q: Constant
[0031]
This equation indicates that interference fringes caused by spherical aberration exist symmetrically with respect to the sharing axis 68 and the vertical line 70 regardless of the reference direction. Spherical aberration does not cause a phase difference in light intensity between two points on the vertical line 70. Similarly, there is no phase difference due to spherical aberration between changes in light intensity at two points located symmetrically with respect to the sharing axis 68.
[0032]
As shown in FIGS. 6A to 6C, the wavefront due to coma aberration can be expressed by the coordinates of the ξ axis and the η axis by the following equation (3).
Фcomma = R (ξ ² + Η ² ) Η (3)
Фcomma: Wavefront function due to coma
R: Constant
[0033]
This equation (3) means that the coma aberration depends on the η direction (referred to as coma direction as necessary). Generally, the frame direction is sharing Different from direction 68. In order to obtain the frame direction, it is necessary to decompose the frame into two components (a first frame component in the sharing direction and a second frame component in the vertical direction 70). Next, the sizes of the first frame component and the second frame component are obtained, and the frame direction is obtained by vector analysis using the magnitudes.
[0034]
When the coma direction coincides with the sharing axis 68, interference fringes due to coma aberration appear symmetrically with respect to the sharing axis 68, as shown in FIG. 6A. This means that the phase difference of the light intensity between two points located on the sharing axis 68 and symmetrically in the center of the symmetric interference fringe depends only on the second coma aberration. On the other hand, the frame direction is sharing When perpendicular to the direction 68, the interference fringes due to coma appear symmetrically with respect to the sharing direction 68 and the vertical direction 70, as shown in FIG. 6C. This means that the phase difference in light intensity between two points located on the vertical line 70 depends only on the first frame component. For reference, FIG. sharing The interference fringes due to coma aberration when the shaft 68 is rotated by 45 ° with respect to the reference direction 70 are shown.
[0035]
7A to 7C show interference fringes due to astigmatism. Astigmatism is expressed in coordinates on the η axis by the following equation (4).
Фastigmatism = Sη ² (4)
Фastigmatism: Wavefront function due to astigmatism
S: Constant
[0036]
This equation (4) means that astigmatism depends only on the direction η. Therefore, when two diffraction images are shared in the ξ direction, no interference fringes appear as shown in FIG. 7C. On the other hand, when the diffraction image is shared in a direction other than the ξ direction, as shown in FIG. 7B, the interference fringes appear to extend parallel to the direction ξ. Also in the η direction sharing In this case, the distance between adjacent interference fringes is minimized as shown in FIG. 7A.
[0037]
Next, referring to FIG. 2 again, how to obtain coma and astigmatism will be described in detail below. Corresponding to this purpose, a plurality of points are set in the sharing area 66 of the sharing pattern 60. The point can be set on the display unit using an appropriate input device 36 such as a keyboard or a mouse. Specifically, the points (B1) and (B2) are on the vertical line 70 at a predetermined distance (L1) from the midpoint (A) on both sides of the midpoint (A) and on both sides of the sharing shaft 68. Selected. Similarly, the points (C1) and (C2) are set on the vertical line 70 at a predetermined distance (L2) from the midpoint (A) on both sides of the midpoint (A). In this embodiment, (L1) is different from (L2), but (L1) may be the same as (L2). Further, the other points (D1) and (D2) are symmetrically spaced on both sides of the sharing axis 68, spaced from the sharing axis 68 (L2), and on one side of the vertical line 70, the distance (L3) from the vertical line 70. ) Is set.
[0038]
Next, a change in light intensity is detected at each of the set points (A), (B1), (B2), (C1), (C2), (D1), and (D2). This is done by detecting the intensity of the signal sent from the corresponding CCD element of the image receiver 28. Subsequently, using the detected intensity change, the phase of the signal or intensity is determined for each point.
[0039]
The phase difference of the light intensity between the points (B1) and (B2) corresponds to the phase difference between two points on the vertical line 70 symmetric with respect to the sharing axis 68. It depends only on focus, spherical aberration, and coma).
[0040]
The phase difference in light intensity between the points (A) and (B1) does not affect the defocusing. This is because the points (A) and (B1) are located on a line perpendicular to the sharing direction, so that the phase difference of the light intensity between them is independent of the second component of coma aberration. This means that the light intensity phase difference between the points (A) and (B1) corresponds to the sum of the first coma component in the coma direction and astigmatism. Note that the distance between the points (A) and (B1) is half the distance between the points (B1) and (B2). This means that the phase difference in light intensity between points (A) and (B1) caused by astigmatism is half of the phase difference between points (B1) and (B2). Therefore, the difference between the phase difference of the light intensity between the points (A) and (B1) and the phase difference of the light intensity between the points (B1) and (B2) is the first frame component in the frame direction. Represents the size of.
The phase difference in light intensity between the points (C1) and (C2) also arises from astigmatism. Since the points (D1) and (D2) are located symmetrically with respect to the sharing axis 68, the phase difference in light intensity between them is independent of defocus, spherical aberration, and the first coma component in the coma direction. Is related to the second coma component in the direction perpendicular to the coma direction and astigmatism. Note that the distance between the points (D1) and (D2) is equal to the distance between the points (C1) and (C2), and therefore the light between the points (D1) and (D2) due to astigmatism. The phase difference in intensity is equal to the phase difference in light intensity between points (C1) and (C2). Therefore, the difference between the phase difference of the light intensity between the points (D1) and (D2) and the phase difference of the light intensity between the points (C1) and (C2) is in the direction perpendicular to the frame direction. Represents the second frame component.
[0041]
Therefore, the magnitudes of the first frame component and the second frame component are expressed by the following equations (5) and (6).
PD1 = | ph (A) -ph (B1) |-| ph (B1) -ph (B2) | / 2 (5)
PD2 = | ph (D1) -ph (D2) |-| ph (C1) -ph (C2) | (6)
PD1: First frame component
PD2: Second frame component
ph (A): phase of light intensity at point A
ph (B1): Phase of light intensity at point B1
ph (C1): phase of light intensity at point C1
ph (C2): phase of light intensity at point C2
ph (D1): phase of light intensity at point D1
ph (D2): phase of light intensity at point D2
Further, the frame direction can be obtained by vector analysis using the phase difference between PD1 and PD2.
[0042]
As described above, coma is the difference between two diffraction images. sharing Evaluation can be performed from the phase difference of a plurality of selected points in the region without reproducing the original wavefront of the sharing interference image.
[0043]
The points (C1) and (C2) are shifted from the points (B1) and (B2), respectively, but these points (C1) and (C2) are located on the points (B1) and (B2), respectively. You may let them.
[0044]
A method for obtaining astigmatism will be described in detail with reference to FIGS. 3A to 3C. FIG. 3A shows a shearing interference image, in which the sharing axis coincides with the reference direction in which the diffraction grating moves. FIG. 3B shows a sharing interference image when the sharing shaft 68 is rotated by an angle θ2 (0 <θ2 <90 °). On the other hand, FIG. Shows another sharing interference image when the sharing axis 68 is rotated by a right angle θ2 (90 °) with respect to the reference direction. In these drawings, (E1) and (E2) indicate points that are located on the vertical line 70, are arranged symmetrically with respect to the sharing axis 68, and are separated from the sharing axis 68 by a predetermined distance (L7).
[0045]
In the case of the present embodiment, the phase difference of the light intensity between the points (E1) and (E2) is expressed by the following formula (7).
PD _R1-R2 = | Ph (E1) -ph (E2) | (7)
PD _R1-R2 : Phase difference of light intensity between points (E1) and (E2)
ph (E1): phase of light intensity at point (E1)
ph (E2): phase of light intensity at point (E2)
[0046]
Using this formula, the magnitude of astigmatism forms an angle other than 90 °, 180 °, or 270 ° with the reference direction, as shown in the following formulas (8), (9), and (10). It can be obtained from two sharing interference images with respect to the two sharing directions X1 and X2.
PD _{X1 (R1-R2)} = | Ph _X1 (E1) -ph _X1 (E2) | (8)
PD _{X2 (R1-R2)} = | Ph _X2 (E1) -ph _X2 (E2) | (9)
here,
PD _{X1 (R1-R2)} : Phase difference between E1 and E2 with respect to direction X1
PD _{X2 (R1-R2)} : Phase difference between E1 and E2 with respect to direction X2
Mastigmatism = PD _{X1 (R1-R2)} -PD _{X2 (R1-R2)} (10)
Where Mastigmatism: Astigmatism magnitude
On the other hand, the method of astigmatism is obtained by the vector analysis using the phase difference from the above formula and the following formulas, that is, formulas (8), (9), and (11).
PD _{X3 (R1-R2)} = | Ph _X3 (E1) -ph _X3 (E2) | (11)
Where PD _{X3 (R1-R2)} : Phase difference between E1 and E2 with respect to direction X3
[0047]
The three directions X1, X2, and X3 should be determined so that at least one of the angles between the directions X1 and X2, X2 and X3, and X3 and X1 is not 90 °, 180 °, and 270 °.
The reason will be described. Specifically, phase difference PD _{X1 (R1-R2)} And PD _{X2 (R1-R2)} The phase difference between and includes only astigmatism other than defocus, spherical aberration, and coma. Also, astigmatism varies depending on the sharing direction. As a result, for example, when sharing in a certain direction, astigmatism does not occur, but another sharing in another direction perpendicular to the certain direction has the closest interference fringe extending perpendicular to the sharing direction. Arise. Therefore, there is no phase difference in the light intensity between the points (E1) and (E2).
[0048]
Therefore, the light intensity phase difference between points (E1) and (E2) caused by astigmatism is selected from two directions that form an angle other than 90 °, 180 °, or 270 ° with the reference direction. It can be obtained by sharing the diffraction image in each of these two directions. The angle is preferably set to 45 ° in order to eliminate direction dependency from the detection result.
[0049]
PD obtained from equations (8) and (9) for these two directions _{X1 (R1-R2)} And PD _{X2 (R1-R2)} Are the same, the third direction X3 passing through the center between the two directions X1 and X2 is specified as a direction in which astigmatism exists or a direction in which no astigmatism exists. Therefore, in order to determine the direction in which astigmatism exists, the third direction is not formed so as to form an angle of 90 °, 180 °, or 270 ° with the above two directions. sharing Find the direction.
[0050]
Therefore, when determining the three directions, it is necessary that at least one of these three directions does not form an angle of 90 °, 180 ° or 270 ° with any one of the remaining two directions. This means that if one of the three directions remains at an angle of 90 °, 180 ° or 270 ° with one of the remaining directions, the two directions of the three sharing directions are the same. sharing This is because the directions coincide with each other, resulting in only two sharing directions.
[0051]
As above sharing Once the direction is decided, sharing The astigmatism magnitude of the lens is obtained from the sum of the two Mastigmatisms according to the above formula for two directions. In addition, the direction of astigmatism is obtained by vector analysis using three phase differences. As described above, the astigmatism of the optical device can be evaluated without obtaining the original wavefront.
[0052]
In the above embodiment, the astigmatism of the lens is obtained using the 0th order and + 1st order diffracted light. However, the + 1st order and −1st order diffracted light or the 0th order and −1st order diffracted light are used. Astigmatism may be obtained by using this. (See FIG. 8) Three diffracted lights (for example, 0th order, + 1st order, and −1st order diffracted lights) are transmitted through the lens 12, and the 0th order, + 1st order diffracted light, 0th order and − The first order diffracted light may be superimposed on different regions of the lens 12. (See FIG. 9) In these cases, spherical aberration and astigmatism can be obtained in the same manner as described above.
[0053]
Also, with respect to the reference direction sharing In order to change the axis, the drive mechanism 48 may be provided with a function of rotating the diffraction grating about the optical axis 22 of the lens 12, but in order to change the sharing direction with respect to the reference direction, the reflection shown in FIG. A type of diffraction plate may be used. The diffractive plate 80 has three diffraction gratings 82, 84 and 86, each of which has a large number of grooves formed in parallel in different directions. For example, the grooves of the diffraction gratings 84 and 86 form + 45 ° and −45 ° with the groove of the diffraction grating 82, respectively. The diffraction plate 80 is supported so as to move in a direction perpendicular to the reference direction and selectively position one of the three gratings 82 to 86 at the focal point of the lens 12.
[0054]
(2) Second embodiment
FIG. 11 shows another system 1A for evaluating the astigmatism of the lens. Instead of the reflection type diffraction grating, the system 1A of this embodiment includes a transmission type diffraction grating 9C, and the light from the lens 12 is in the order of 0th order, ± 1st order, ± 2nd order,. Diffracted into diffracted light. Another lens 92 is provided in the vicinity of the diffraction grating 90 and away from the lens 12, and the 0th order and the + 1st order, the 0th order and the −1st order, the + 1st order and the −1st order, or the 0th order The diffracted light of ± 1st order is sent to the lens 92, and the light passes through a part of the opening of the lens 92 or the pupil. sharing I have to do it. This can be adjusted by the diffraction angle of the diffraction grating 90 determined by the wavelength of the incident light and the groove interval of the diffraction grating 90. The image receiving unit 28 is arranged to receive light from the lens 92. The transmission type diffraction grating 90 is used at the location of the reflection type diffraction grating, and it is not necessary to provide a light separation device (beam splitter).
[0055]
The operation will be described. The diffraction grating 90 is moved in the reference direction by an appropriate drive mechanism. The light from the light source 14 or the laser 16 passes through the modulator 18 and the lens and forms an image on the diffraction grating 90. The light is diffracted by the diffraction grating 90 into diffracted light on the order of 0th order, ± 1st order, ± 2nd order,. In particular, diffracted light in the order of 0th order and + 1st order, 0th order and −1st order, + 1st order and −1st order, or 0th order and ± 1st order is sent to the image receiving unit 28 via the lens. The image receiving unit 28 creates a signal corresponding to the received image and sends the signal to the signal processor 30. The signal processor converts the signal into an image signal, and an image received using the image signal is displayed on the display unit 34 of the display unit 32. The displayed image is then used to evaluate lens aberrations as described above. In this case, the sharing direction is changed by the driving mechanism 48 or by using a transmission type diffraction grating.
[0056]
As shown in FIG. 12, a transmissive diffractive plate 93 similar to the reflective diffractive plate shown in FIG. 10 may be used. The transmission type diffraction plate 93 has three diffraction gratings, and each of them has a large number of slits formed in a specific direction. For example, like the reflection type diffraction plate shown in FIG. 10, the grooves of the second and third diffraction gratings form + 45 ° and −45 ° with the grooves of the first diffraction grating.
[0057]
(3) Third embodiment
FIG. 13 shows another form of a part of the system 1B having the diffraction unit 94. The diffraction unit 94 has a pair of first and second transmission type diffraction gratings 941 and 942. The first and second diffraction gratings 941 and 942 are formed with grooves at predetermined intervals, and the input light is excluded from the 0th order diffracted light, ± 1st order, ± 2nd order,... Can be diffracted into diffracted light of the order of. The first and second diffraction gratings 94 and 94 are disposed parallel to and adjacent to the lens 12 and perpendicular to the optical axis 22 of the lens 12 with their grooves oriented in a certain direction. The second diffraction grating 942 is supported by the support unit 96 and moves in the reference direction 50 perpendicular to the optical axis 22. The first and second diffraction gratings 941 and 942 are supported by a drive mechanism 98 so that they can rotate together about the optical axis 22.
[0058]
In the operation of the system 1B configured as described above, the second diffraction grating 942 moves in the reference direction 50 perpendicular to the optical axis 22, while light is transmitted through the lens and collimated. The collimated light is transmitted through the first diffraction grating 941 where it is diffracted into diffracted light on the order of ± 1st order, ± 2nd order,.
[0059]
Next, the diffracted light of the + 1st order and the −1st order are diffracted by the second diffraction grating 942 into diffracted lights of the order of ± 1st order, ± 2nd order,.
[0060]
The -1st order diffracted light from the second diffraction grating and obtained from the + 1st order diffracted light from the first diffraction grating 941 extends parallel to the optical axis 22. Similarly, light obtained from + 1st order diffracted light from the second diffraction grating 942 and obtained from -1st order diffracted light from the first diffraction grating 941 is also parallel to the optical axis 22. extend. However, + 1st order and −1st order diffracted light from the second diffraction grating 942 and light obtained from the −1st order and + 1st order diffracted light from the first diffraction grating 941 is light. It is slightly shifted in the direction perpendicular to the axis 22 and forms a sharing interference image. This sharing interference image is received by the image receiving unit 28 and used for evaluating the aberration of the lens 12 as described above. The diffraction gratings 941 and 942 are simultaneously rotated about the optical axis 22 in order to evaluate astigmatism.
This configuration is very effective for such a system because the optical structure of the system can be considerably simplified.
[0061]
The diffraction gratings 941 and 942 rotate around the optical axis 22 to change the direction of the sharing axis. As shown in FIG. 14, the diffraction gratings 941 and 942 having three diffraction gratings are transmissive diffraction plates 941. It may be replaced with 'and 942'. In this case, a mechanism for moving the diffraction unit is necessary to exchange the diffraction grating.
[0062]
(4) Fourth embodiment
FIG. 15 shows a system 100 that can evaluate and correct coma and astigmatism of an optical head 101 used with an optical device such as a DVD. The optical head 1010 has a laser source 102 such as a laser generator for emitting light 104 such as a laser beam. The emitted light 104 is collimated by a collimator lens 106. Next, the collimated light 104 is reflected by the beam splitter 108 and the mirror 110 and sent to the objective lens 112, where it is imaged on the reflective diffraction grating 114 via the transparent cover plate 114. The diffraction grating 116 is moved in the direction indicated by reference numeral 118 by the drive mechanism 120. In order to evaluate astigmatism, the diffraction grating 116 is rotated by the mechanism 120 around the optical axis of the objective lens 112 in the direction indicated by reference numeral 112. Diffracted and reflected light (particularly 0th and + 1st order, 0th and −1st order, or + 1st and −1st order diffracted light) is used for the objective lens 112, the mirror 110 and the beam splitter 108. Is sent to the image receiving unit 124. The image receiving unit 124 has small light receiving elements of a photoelectric conversion element (CCD), and these elements convert the received image into a series of image signals. Next, the image signal is sent to the signal processor 126 where it is processed into an image signal of a sharing interference image displayed on the display unit 128. Using the displayed shearing interference image, the coma and astigmatism of the system are evaluated by the process described above.
[0063]
The coma is corrected by adjusting the angle between the axis of the objective lens 112 and the optical axis of the light from the light source and / or the position of the light source 102 on the XY plane extending perpendicular to the optical axis. Is done. For this purpose, the objective lens 112 is supported by a mechanism 130 that can adjust the angle or inclination of the objective lens 112. The light source 102 is supported by a mechanism 132 for moving the position in the XY plane.
[0064]
On the other hand, astigmatism is corrected by moving the collimator lens 106 in the axial direction (that is, the Z direction) and adjusting the parallelism of the collimated light. Therefore, the collimator lens 106 is supported by a mechanism 134 that can move the collimator lens 106 in the Z direction.
[0065]
Alternatively, coma can be adjusted by moving the collimator lens 106 in the XY plane and / or by changing the reflection angle of the l optical elements such as the beam splitter 108 and the mirror 110. The astigmatism can be adjusted by moving the light source 102 and / or the objective lens 112 in the axial direction (that is, the Z direction) or by moving the position of the beam splitter 108.
Further, the diffraction grating 116 may be replaced with a reflection type diffraction plate shown in FIG. In this case, the diffractive plate moves perpendicular to the direction 116 to change the sharing direction.
Further, the diffraction grating and the cover glass may be replaced with a part of the optical disk.
Furthermore, although the cover glass 114 is provided on the diffraction grating 116, it may be omitted. In this case, the system is designed so that the objective lens properly images light onto the diffraction grating 116.
The system can be applied to adjustment of any optical apparatus such as a laser beam recording apparatus (LBR), a laser processing apparatus, or a laser microscope in which light is formed as a light spot on an object.
[0066]
(5) Fifth embodiment
FIG. 16 shows another system 150 for evaluating and correcting coma and astigmatism of an optical head 152 used with an optical device such as a DVD. The optical head 152 is fixedly supported by the support mechanism 153 and includes a laser source 154 such as a laser generator that emits light 156 such as a laser beam. Further, a collimating lens 158 for collimating the emitted light 156 and an objective lens 160 for forming an image of the collimated light 156 on a transmission type diffraction grating 162 having a large number of parallel slits are provided. . The diffraction grating 162 may be replaced with a transmission type diffraction plate. Further, although a transparent cover glass 164 is provided on one side of the diffraction grating 162, it can be omitted.
[0067]
The position of the diffraction grating 162 relative to the head 152 is adjusted to satisfy the positional requirements required for the optical head and the optical disk in the product.
[0068]
The diffraction grating 162 is moved in the direction indicated by reference numeral 166 by the drive mechanism 168. Further, in order to evaluate astigmatism, the diffraction grating 162 is rotated by the mechanism 168 in the direction indicated by reference numeral 170 around the optical axis of the objective lens 160.
The light 156 imaged on the diffraction grating 162 is diffracted into diffracted light on the order of ± first order, ± second order,. Next, the diffracted light is sent to another lens 172. In the present embodiment, the 0th-order and + 1st-order diffracted light or the 0th-order and −1st-order diffracted light are transmitted to the lens 172 and partially overlapped by the lens 170 to form a sharing pattern. . This sharing pattern is received by an image receiving unit 174 having a small light receiving element of a photoelectric conversion element (CCD), which converts the received image into a series of image signals. Next, the image signal is sent to a signal processor 176 where it is processed into an image signal of a sharing interference image displayed on the display unit 178. Subsequently, the coma aberration and astigmatism of the head 152 are evaluated by the above-described phase shift method using the displayed shearing interference image.
[0069]
The coma aberration can be corrected by adjusting the angle of the objective lens 160 with respect to the optical axis and / or by moving the light source 154 in a plane perpendicular to the optical axis (that is, the XY plane). Therefore, the support mechanism 153 that supports the head 152 is designed to move so as to change the angle of the objective lens 160. The light source 154 is supported by another mechanism 182 that can move the light source 154 in the X direction and the Y direction.
On the other hand, astigmatism can be corrected by moving the collimator lens 158 along the optical axis, thereby adjusting the parallelism of the light sent to the objective lens 160.
[0070]
(6) Sixth embodiment
FIG. 17 shows another system 200 for evaluating and correcting coma and astigmatism of an optical head 201 used with an optical device such as a DVD. The optical head 201 is fixedly supported by a support mechanism 202 and has a light source 203 such as a laser generator for emitting light 204 such as a laser beam. The head 201 is provided with a collimator lens 205, a beam splitter 206, and an objective lens 208, and the light 204 emitted from the light source 203 is sent through the collimator lens 205 and the objective lens 208 and imaged on the mirror 210. The position of the mirror with respect to the head 201 is adjusted so as to satisfy the positional conditions required for the optical disk and the optical head 201 in an actual product. The mirror 210 may be replaced with a part of the optical disk, or a cover glass may be provided on the surface facing the objective lens 208.
[0071]
The light 204 reflected by the mirror 210 is sent to a diffraction grating unit denoted by reference numeral 212 through an objective lens 208 and a beam splitter 206. The light 204 sent to the diffraction grating unit 212 is collimated.
[0072]
The diffraction grating unit 212 has two opposing transmission type diffraction gratings 214 and 216 arranged in parallel as shown in FIG. Therefore, the collimated light 204 is diffracted by the first diffraction grating into diffracted light on the order of ± 1st order, ± 2nd order,. The diffracted light is similarly diffracted into diffracted light on the order of ± 1st order, ± 2nd order,... By the second diffraction grating.
[0073]
The first diffraction grating 214 is supported by the mechanism 220 so as to move in the direction indicated by reference numeral 218 perpendicular to the light entering the diffraction grating. The first and second diffraction gratings 214 and 216 are supported by a mechanism 220 so as to be rotatable about the optical axis.
[0074]
Each of the diffraction gratings 214 and 216 may be replaced with a transmission type diffraction plate provided with the three diffraction gratings described above. In that case, the diffraction plate moves perpendicular to the optical axis to change the diffraction grating.
[0075]
Diffraction gratings 214 and 216 obtain + 1st order diffracted light from the second diffraction grating 216 obtained from -1st order diffracted light from the first diffraction grating 214, and The -1st order diffracted light of the second diffraction grating 216 obtained from the + 1st order diffracted light partially overlaps to form a sharing pattern. The diffraction gratings and their positions are determined so that the + 1st order and −1st order diffracted lights from the second diffraction grating 212 move in parallel with each other.
[0076]
Next, the image is received by the image receiving unit 222 having a small image receiving element in the photoelectric conversion element (CCD). The image receiving element converts the received image into a series of image signals. The image signal is sent to the signal processor 224, where it is processed into an image signal of a sharing interference image displayed on the display unit 226. Then, the coma aberration and astigmatism of the head 201 are evaluated by the above-described phase shift method using the displayed sharing interference image.
[0077]
The coma aberration can be corrected by adjusting the angle of the objective lens 208 with respect to the optical axis and / or by moving the light source in a plane perpendicular to the optical axis (that is, the XY plane). For this purpose, the support mechanism 202 that supports the head 201 is designed to move to change the angle of the objective lens 208, and the light source 203 is another mechanism that can move the light source 203 in the X and Y directions. 228 is supported.
[0078]
On the other hand, astigmatism can be corrected by moving the collimator lens 205 along the optical axis, thereby adjusting the parallelism of the light sent to the objective lens 208. For this purpose, the collimator lens 205 is supported by a mechanism 230 that can move the collimator lens 205 along the optical axis.
[0079]
(7) Seventh embodiment
FIG. 18 shows another system 250 for evaluating and correcting coma and astigmatism of an optical head 252 used with an optical device such as a DVD. The optical head 252 is fixedly supported by a support mechanism 254 and includes a light source 256 such as a laser generator that emits light 258 such as a laser beam. The head 252 is provided with a collimator lens 260 and an objective lens 262 so that light 258 emitted from the light source 256 is sent through the collimator lens 260 and the objective lens 262.
[0080]
The system has another lens 264 that modulates the light sent from the objective lens 262 into collimated light. The collimated light is then sent to the grating unit 266.
[0081]
The grating unit 266 has two opposing transmission type diffraction gratings 268 and 270 arranged in parallel as shown in FIG. The collimated light 258 is diffracted by the first diffraction grating into diffracted light on the order of ± 1st order and ± 2nd order. Next, the diffracted light is diffracted by the second diffraction grating into diffracted light on the order of ± 1st order, ± 2nd order, and so on.
[0082]
The first diffraction grating 268 is supported by the mechanism 272 so as to move in the direction indicated by reference numeral 274 perpendicular to the light incident on the diffraction grating. The first and second diffraction gratings 268 and 270 are supported by a mechanism 272 so that the first and second diffraction gratings 268 and 270 can rotate around the optical axis indicated by reference numeral 276.
[0083]
Each of the diffraction gratings 268 and 270 may be replaced with a transmission type diffraction plate having the three diffraction gratings described above. In this case, the diffraction plate moves perpendicular to the optical axis in order to exchange the diffraction grating.
[0084]
According to these two diffraction gratings 268 and 270, the diffracted light of the + 1st order obtained from the second diffraction grating 270 and the diffracted light of the −1st order of the first diffraction grating 268 before that. And -1st order diffracted light obtained from the second diffraction grating 270, and light obtained from the + 1st order diffracted light of the first diffraction grating 268 before that. Partially superimposed to form a sharing pattern. The diffraction gratings and their positions are determined so that the + 1st order and −1st order diffracted lights obtained from the second diffraction grating 270 move in parallel with each other.
[0085]
Next, the sharing pattern is received by an image receiving unit 278 having small image receiving elements in a photoelectric conversion element (CCD). The image receiving element converts the received light into a series of image signals. Next, the image signal is sent to the signal processing unit 280, where it is processed into an image signal of a sharing interference image displayed on the display unit 282. Thereafter, the coma aberration and astigmatism of the head 252 are evaluated by the above-described phase shift method using the displayed sharing interference image.
[0086]
The coma aberration can be corrected by adjusting the angle of the objective lens 262 with respect to the optical axis and / or by moving the light source 256 in a plane perpendicular to the optical axis (that is, the XY plane). For this purpose, the head 252 is supported by a mechanism 282 that can change the angle of the objective lens 262. The light source 256 can move the light source 256 in the X direction and the Y direction. Supported by another mechanism 284.
[0087]
Astigmatism can be corrected by moving the collimator lens 262 along the optical axis, thereby adjusting the parallelism of the light sent to the objective lens 262. Therefore, the collimator lens 260 is supported by a mechanism 286 that can move the collimator lens 260 along the optical axis.
[0088]
As described above, according to the method and apparatus of the present invention, the sharing pattern can be formed with a simple configuration. Further, coma and astigmatism can be evaluated and corrected by obtaining the phase difference of the light intensity between a plurality of points in the sharing region without obtaining the original wavefront. Furthermore, it is not necessary to enlarge the sharing pattern at high magnification, so that the target image, or sharing pattern, can be easily located in the field of view of the receiver, so that the system can very accurately correct aberrations. Can be requested.
[Brief description of the drawings]
FIG. 1 shows an apparatus for evaluating aberrations of an optical element according to a first embodiment of the present invention.
FIG. 2 shows a shearing interference image of two diffracted lights and a plurality of points in a sharing region for obtaining coma aberration of an optical element.
FIGS. 3A to 3C show two sharing interference images and a plurality of points in a sharing region for obtaining astigmatism of an optical element.
4A to 4C show wavefronts caused by defocus. FIG.
FIGS. 5A to 5C show wavefronts caused by spherical aberration. FIGS.
6A to 6C show wavefronts caused by coma.
7A to 7C show wavefronts caused by astigmatism.
FIG. 8 shows another apparatus for evaluating aberrations of an optical element according to another embodiment of the present invention.
FIG. 9 shows another apparatus for evaluating aberrations of an optical element according to the present invention.
FIG. 10 shows a grating plate with three different gratings.
FIG. 11 shows another apparatus for evaluating aberrations of another optical element according to the present invention.
FIG. 12 shows another apparatus for evaluating aberrations of an optical element according to the present invention.
FIG. 13 shows another apparatus for evaluating aberrations of an optical element according to the present invention.
FIG. 14 shows another apparatus for evaluating aberrations of an optical element according to the present invention.
FIG. 15 shows an apparatus for evaluating and correcting aberrations of optical elements according to the present invention.
FIG. 16 shows another apparatus for evaluating and correcting aberrations of optical elements according to the present invention.
FIG. 17 shows another apparatus for evaluating and correcting aberrations of optical elements according to the present invention.
FIG. 18 shows another apparatus for evaluating and correcting aberrations of optical elements according to the present invention.
FIG. 19 is a conventional apparatus for evaluating the aberration of an optical element.
FIG. 20 is another conventional apparatus for evaluating the aberration of an optical element.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Optical system, 10 ... Optical apparatus, 12 ... Lens, 14 ... Light source, 18 ... Transformer, 24 ... Beam splitter, 26 ... Light, 28 ... Image receiving part, 30 ... Signal processor, 32 ... Display unit, 34 ... Display unit, 40 ... diffraction grating, 48 ... drive mechanism.

Claims (4)

A method for evaluating coma aberration of an optical element,
(A) transmitting light through the optical element;
(B) diffracting the light with a diffraction grating to obtain first and second diffracted lights having different diffraction orders; and (c) forming the sharing interference image by superimposing the first and second diffracted lights. And a process of
(D) obtaining a plurality of points in the sharing interference image,
The above points include
A first point that is a midpoint of a first line connecting the axes of the first and second diffracted beams;
A second point located on a second line across the first line at the first point;
A third point located on the second line and symmetrically with respect to the first line from the second point;
Fourth and fifth points located on the second line and symmetrically on both sides of the first line and arranged at a predetermined distance from the first line;
The are disposed on both sides of the first line, it includes the sixth and seventh points being spaced by the predetermined distance from the first line,
The above method is also
(E) determining the light intensity of the first to fifth points;
(F) moving the diffraction grating in a direction perpendicular to the optical axis of the optical element to change the light intensity of the first to fifth points in the sharing interference image;
(G) obtaining a phase of light intensity at each of the first to seventh points;
(H) evaluating the coma aberration of the optical element from the phase at the first to seventh points, and evaluating the aberration of the optical element. The step (h)
A first phase difference Ph (1) of light intensity between the first and second points;
A second phase difference Ph (2) of light intensity between the second and third points;
A third phase difference Ph (3) of light intensity between the fourth and fifth points;
Obtaining a fourth phase difference Ph (4) of light intensity between the sixth and seventh points,
The step (h)
Phase difference = | Ph (1) −Ph (2) | / 2
Obtaining the magnitude of coma from the phase difference obtained from
Phase difference = | Ph (4) −Ph (3) | / 2
The method of evaluating an aberration of an optical element according to claim 1, further comprising: determining a coma aberration direction from the phase difference obtained from the above. A method for evaluating astigmatism of an optical element , comprising:
(A) transmitting light to the optical element;
(B) diffracting the light with the diffraction grating to obtain first and second diffracted lights having different diffraction orders;
(C) superposing the first and second lights to form a shearing interference image;
(D) First and second points in the sharing interference image, the first and second points being on a line crossing the center of the line connecting the centers of the first and second diffracted beams. Detecting the light intensity at a point located symmetrically with respect to the line connecting the centers,
(E) moving the diffraction grating in a direction perpendicular to the optical axis of the optical element to change the light intensity of the first and second points in the sharing interference image;
(F) performing a step of obtaining a phase difference of light intensity between the first and second points,
At least one of 90 ° of the angle between, 180 °, with respect to three directions not 270 °, to evaluate astigmatism of the optical element from the phase difference obtained each aberration evaluation method of the optical element.   4. The aberration evaluation method for an optical element according to claim 3, wherein a magnitude of the astigmatism is obtained from the phase difference in two of the three directions.

Images